A better solution is just get on and build extra runways at KK and LL and probably in time GW and SS.
There is no point maintaining and staffing an entire airport on the off chance.
An extra runway at KK would have solved the closure issues and the GW security threat is a relatively scarce occurrence.
The only thing that's going to happen at Manston is the opportunity to buy a house on Aerodrome Drive, or maybe Control Tower Way